The present invention relates to a water-repellent metal oxide film coated on a glass substrate and a method of forming the film. The film forming method belongs to the sol-gel process using a solution of a metal alkoxide. The water-repellent coating is suitable for application, for example, to vehicular, marine, aircraft and architectural window glasses which are to be exposed to rain water.
It is well known to coat a glass plate surface with a hard and abrasion-resistant film of a metal oxide such as silicon oxide. The metal oxide film can be formed, for example, by the sol-gel process using a solution of an organic metal compound such as a metal alkoxide.
Hitherto, there have been proposals to apply, for example, a compound having a fluoroalkylsilane group or dimethylsiloxane to a glass plate surface for the purpose of improving water-repellency of the glass plate.
For example, JP-A-58-122979 and JP-A-58-172245 disclose a water and oil repellent which has a silane compound containing a polyfluoroalkyl group or a partially hydrolyzed condensation product of the silane compound, and an alkoxysilane compound or a halogenosilane compound. The silane compound and the condensation product have a group, --CON(R.sup.2)--Q-- or --SO.sub.2 N(R.sup.2)--Q--, wherein "Q" represents a bivalent organic group and "R.sup.2 " represents a hydrogen atom or a lower alkyl group.
JP-A-58-167448 discloses a metal oxide film which is coated on a glass substrate and has a thickness not greater than 1 .mu.m. The film is formed by applying to the glass substrate a silane compound containing a polyfluoroalkyl group or a partially hydrolyzed condensation product of the silane compound.
JP-A-64-68477 discloses a water-repellent and durable metal oxide film coated on a steel plate. The film is formed by applying an alcohol solution to the steel plate and by heating the coated steel plate at a temperature not lower than 100.degree. C. The alcohol solution contains not less than 0.05 wt % of at least one of an alkoxide, a monomethylalkoxide and an acetylacetonato of a metal selected from the group consisting of Al, Zr, Ti, Si, W, Ce, Sn and Y, and a fluoroalkylsilane of which concentration is from 0.005 to 0.30 mol % of the total amounts of the at least one of the alkoxide, the monomethylalkoxide and the acetylacetonato.
However, the above-mentioned proposals have the following drawbacks.
It is difficult to apply repellent solutions according to the above-mentioned proposals to a glass substrate, for example, through brushing by hand. That is, the thickness of films coated on the glass substrate tend to become uneven when the repellent solution is applied to the glass substrate through brushing by hand. Furthermore, the abrasion resistance of the films is still unsatisfactory.